"Peptides containing 1,2,3,4-tetrahydroquinoline-2-carboxylic acids" are described by G. P. Zecchini et al in J. Heterocyclic Chem. 16, 1589 (1979), for the cyclization with acetic anhydride, to form "1H,3H,5H-oxazolo[3,4-a]quinolin-3-one derivatives". Also, 1-(mercapto- or acylthioalkanoyl)-1,2,3,4-tetrahydroquinoline-2-carboxylic acids, and salts thereof, are known, e.g., according to DOS No. 3,004,370, as possessing antihypertensive activity. Surprisingly it was found that either by introduction of another aminoacid moiety into the former mono-peptides, or by exchange of the latter mercaptoalkanoyl moiety by a dipeptide chain, superior antihypertensive agents are obtained.